In certain existing data processing networks magnetic tape storage equipment is being used in an inefficient and probably obsolescent manner. After recording information on tape reels at one station, the reels are removed for shelf storage and subsequently manually carried to and mounted at another station where another data processing system processes the recorded data. Many telephone message accounting and call billing networks operate in this manner.
This is quite inefficient by comparison to systems which employ more modern equipment to achieve similar functions, e.g. large capacity disk storage equipment for the data storage function, and telecommunication equipment for functions comparable to the manual transportation of tapes between recording stations, shelf storage facilities and processing stations.
However, direct adaptation of such networks to interface with more modern storage and communication equipment, e.g. by redesign of interfacing equipment and software at the recording and processing stations, is considered too costly and impractical. A more practical expedient, which forms the environmental basis of the present invention, is to link existing recording and processing station equipment "plug-compatibly" to upgraded storage and communication media through an electronically simulated adaptation interface and a "linking" minicomputer (or possibly microcomputer) system. Desirably, the simulating adaptation equipment should make the substitution appear to be plug compatible to the equipment and software reliant on magnetic tape storage; i.e. it should directly emulate the real time signalling processes of magnetic tape storage equipment relative to the recording and processing stations of the network, and otherwise operate in complete "transparency" to the existing (unaltered) parts of the network.
A problem encountered in respect to "electronically" emulating signalling processes of magnetic tape storage equipment is that information transferred at interfaces to such equipment usually consists of variable length trains of data bytes iterated (in byte serial bit parallel form) at a first byte clocking rate, and each train is terminated by a cyclic redundancy check (CRC) character having a slower second clocking rate. For verification purposes, the emulating adapter equipment must "echo" the received data and CRC bytes back to the source equipment, in the order of their reception and with the same byte clocking rates; and this must occur after a predetermined delay (which in the real tape storage equipment is associated with the physical movement of the tape between recording and reproducing heads).
Consequently, an emulating adapter operating electronically should be able to buffer at least portions of the received information train, retransmit buffered data after a predetermined delay, determine precisely when the last byte of data is being retransmitted, switch its internal transmission clocking reference from a first source associated with data byte retransmission to a second source associated with CRC retransmission and then retransmit the CRC byte. Since the number of data bytes is variable, determination of the time of last data byte retransmittal is not a simple logical task (even if reception of that byte is explicitly marked by a control signal from the source equipment).
Furthermore, in the replaced magnetic tape storage equipment information usually is recorded in the NRZI mode and the action of resetting magnetization levels at the bit recording heads after the recording of the CRC character, produces a state of magnetization on the tape which represents a longitudinal redundancy check (LRC) character. This character normally is picked up at the reproducing head after retransmission of the CRC character and transmitted to the origin station equipment contiguous in time to the CRC character and at the same "second" clocking rate as the CRC character. An additional problem in respect to the present simulation is to generate, buffer and transfer an equivalent LRC character by logical electronic means.
Objects of the present invention are to provide "plug compatible" adaptation means for electronically simulating communication operations of magnetic tape storage equipment at an interface between unaltered data recording and processing stations of an existing data processing network and a mini or micro-computing system serving relative to such stations as a replacement for magnetic tape storage equipment. Ancillary objects are to provide simple and cost effective electronic means for solving the foregoing problems in respect to verification retransmittals of varied length trains of data and CRC bytes, and in respect to generation and transmittal of LRC characters.